This invention relates to the measurement of range of a moving target and, more particularly, to the performance of such measurement by the use of bearing data relative to a moving vehicle.
The measurement of the position of a moving target relative to a moving vehicle, such as an aircraft or a ship, is frequently performed by the use of radar or sonar. Data may be received by the use of an active mode or a passive mode of operation of the radar or sonar. In the active mode, radiant energy is transmitted towards the target followed by a reception of the echo, the elapsed time and any modulation of the transmitted signal providing the target data. In the passive mode, there is no transmission of radiant energy towards the target, and the receivers of the radar or sonar are tuned to receive radiant signals that may emanate from the target, with all target data being based only on the measurement of bearing of the target relative to the moving vehicle.
The passive mode of operation is advantageous for, and used in, situations wherein it is desirable that the measurement operation should go unnoticed by third parties and by personnel associated with the moving target. Previous methods of passive target location have been accomplished by vectoring and referencing a sensor, or by referencing a plurality of omnidirectional sensors, and comparing their respective signals. The target to be detected may be either a stationary or moving emitter of radiant energy.
Different forms of passive detection systems have been disclosed such as, by way of example, those of U.S. Pat. No. 2,940,076, issued June 7, 1960 to T. B. Bissett et al, and U.S. Pat. No. 3,304,409, issued Feb. 14, 1967 to C. Snowdon et al. In the Bissett system, a plurality of antennae receives signals from a moving vehicle, the signals being compared to provide directional data. In the Snowdon system, target range is computed from bearing data as well as the first and second derivatives of bearing angle.
Another method is a variation of a technique that has been used by sailors since antiquity to avoid collision. Basically, the observing or test craft detects energy waves and modifies its own course until the measured bearing to the target has not changed with time. At this point, the test craft undergoes a further maneuver whereupon the range may then be calculated. It is noted that this technique may be ineffective against targets moving at a velocity greater than the velocity of the observer since a zero bearing rate is not always attainable. Further limitations develop when the target is a noncontinuous emitter, and when the measurement system is required to locate more than one target at a time.
As an example of a system requiring continuous reception of target emission for passive angular measurements, there is disclosed a passive ranging system in the U.S. Pat. No. 3,982,246, issued Sept. 21, 1976 to B. H. Lubar. The Lubar system is rather complex in requiring a stabilized platform, a computer and apparatus responsive to angular rates of rotation and acceleration. Generally speaking, systems and methods of the prior art have involved the use of a variety of optical, electromagnetic and accoustic sensors, as well as the use of cooperating ground-based receiving and transmitting stations. Many such systems also require a plurality of cooperating sensor units and cross-correlation equipment.
Much of the foregoing problem attendant the passive ranging methodology and systems has been overcome by the teachings of a more recent U.S. Pat. No. 4,179,697 which issued Dec. 18, 1979 in the name of Martin Golinsky, the inventor herein. The passive ranging method disclosed in the Golinsky patent is incorporated herein by reference and demonstrates the measurement of the range of a target aircraft relative to a measuring aircraft. The method involves only a single measuring aircraft, and is accomplished by flying the measuring aircraft along a closed curved path while simultaneously performing a sequence of passive bearing measurements of the target aircraft relative to the measuring aircraft. There results a geometry of intersecting rays which is readily solved by a mathematical operation to provide the target position.
However, a problem still remains in that the foregoing requirement of the nonlinear measurement path, flown by the measuring vehicle, may not be practical in many situations wherein the measuring vehicle may be required to travel in a straight line so as to complete its mission. Under such a constraint, the method of the foregoing Golinsky patent would not be available for the passive measurement of target position.
It is also noted that, while the techniques of passive ranging are most frequently described with reference to the use of radar for tracking aircraft, they are equally applicable to the tracking of ships and spacecrafts. In the ensuing description of the invention, the invention will be described with reference to the tracking of aircraft, it being understood that the terminology and methodology are equally applicable to ships employing sonic sensors and to satellites employing optical sensors. It is to be noted that certain assumptions utilized in the aforementioned Golinsky patent are valid here. Specifically, the target is not maneuvering, and the test aircraft is at a long range from the target so that they may be considered to be in the same horizontal plane.